Frigorific separation process of gas mixture by liquefying and rectifying



Oct. 9, 1956 Filed Oct. 10, 1951 A. ETIENNE FRIGORIFIC SEPARATIONPROCESS OF GAS MIXTURE BY LIQUEFYING AND RECTIF'YING 3 Sheets-Sheet 1@ct. 9, 1956 ETIENNE 2,765,637

A. FRIGORIF'IC SEPARATION PROCESS OF GAS MIXTURE BY LIQUEFYING ANDRECTIFYING Filed Oct. 10, 1951 5 Sheets-Sheet 2 Oct. 9, 1956 ET|ENNE2,765,637

FRIGORIFIC SEPARATION PROCESS OF GAS MIXTURE BY LIQUEF'YING ANDRECTIFYING Filed Oct. 10, 1851 3 Sheets-Sheet 3 Fig.3.

2,75,37 Patented Oct. 9, 1956 ice FRIGORIFIC SEPARATION PROCESS OF GASMIXG TU RE BY LIQUEFYING AND RECTIFY- IN Alfred Etienne, Paris, France,assignor to LAir Liquide, Societe Anonyme pour lEtude et lExploitationdes Procedes Georges Claude, Paris, France Application October 10, 1951,Serial No. 250,630 Claims priority, application France October 30, 195010 Claims. (Cl. 62-175.5)

The present invention relates to improvements in the cold fractionationof gaseous mixtures. More particularly it relates to the case where thesole fractionation or each of the successive fractionations delivers twofractions and is operated in two successive rectification columnsWorking at different pressures and separated by a common heat transferunit functioning as a condenser for the vapors in the higher pressurecolumn and as a boiler for the liquid in the lower pressure column.

A first object of this invention is to minimize the operating powersupply in the cases where the separation of the constituents is madeuneasy, either by the narrow ditference of their boiling points, or by apuzzling ratio between the respective amounts of the constituents,especially when the more volatile one makes only a low percentage of thewhole mixture.

A further object is to effect economically a sharp fractionation, thatis to say to obtain pure constituents not or little contaminated by eachother.

Another object is to avoid the use of auxiliary refrigeration cyclesusing pure separated constituents expanded with or without performanceof external work.

In the following, the two rectification columns will be termed as highpressure column" and low pressure column disregarding the absoluteoperation pressures. The two fractions between which the incomingmixture is separated in these two columns will be termed as low boilingfraction and high boiling fraction being understood that the firstmentioned is the one, the boiling point of which is the least expressedin Kelvin degrees.

The improvements comprising this invention reside in the followingfeatures:

A liquid resulting from the partial condensation of a compressed gaseousmixture is expanded and so partly vaporized. The resulting mixture ofgas and liquid enters the low pressure column from the bottom of whichthe high boiling fraction is withdrawn either in a liquid condition orin a gaseous one. The gases issuing from the top of the low pressurecolumn are compressed and led to the bottom of the high pressure columnwherein they are rectified. From the top of the high pressure columnissues the low boiling fraction in a gaseous state. The upper part ofthe high pressure column is cooled, as it is conventional, by theboiling of the liquid in the bottom of the low pressure column containedin a common heat transfer unit. The liquid issuing from the bottom ofthe high pressure column is expanded and used as scrubbing liquid at thetop of the low pressure column.

From the above summary of the invention appear the following differenceswith the prior art:

(a) The feed is only on the low pressure column instead of being on thehigh pressure or of being divided between both columns.

(b) The two fractions resulting from the separation are Withdrawn, onefrom each column, instead of being both withdrawn from the low pressurecolumn.

() An intermediate product is led from the low pressure column to thehigh pressure column, being therefore compressed.

It appears that this process overcomes the difiiculty that is raisedwhen a mixture to be fractionated holds such a low percentage of its lowboiling component that the amount of washing liquid is not suflicient toperform with the conventional devices a separation of this component ina good purity condition. A similar difiiculty appears when the twocomponents to be separated have closely boiling points, and iseliminated by the present invention. It is to be noted that it is knownto increase in such cases the amount of washing liquid by supplying anextra amount of cold through the compression, and further expansion,preferably with performance of external work, of one of the twoseparated components. The present invention fulfills the same purpose bycompressing an intermediate mixed product, the previous purification ofwhich is consequently not needed.

It is to be emphasized that a less power supply than in the conventionaldevices is needed, owing to the fact that the feeding is performed onthe low pressure column Where from one of the fraction is directlywithdrawn, and that consequently only a part of the incoming mixture hasto be compressed at the high pressure.

complex gaseous mixture, this separation may have occurred by theconventional rectification or partial condensation.

As an example, this complex gaseous mixture is a natural gas; its higherboiling component gases are condensed by compression followed bycooling, and it is this condensate which is fractionated according tothe present invention, the gaseous residue of the cooling being thenitself either fractionated or used as a fuel.

An another example, the initial mixture is air which embodiment of myinvention.

Fig. 2 is a diagrammatic view of another embodiment of my invention,applied to the fractionation of a mineral oil gas.

Fig. 3 is a diagrammatic View of an apparatus separ ating atmosphericair into nitrogen, oxygen and an argonr1ch fraction operating accordingto my invention.

According to Fig. l, the compressed mixture to be equal to, or higherthan that at Which occurs the partial liquefaction in the exchanger E2.At the base of column Cl, is removed, in a liquid condition, throughline F and regulation valve B5, a fraction containing the less volatileconstituents of the initial mixture; this fraction comes out of theapparatus at U, after having, by its vaporization and its warming toroom temperature, supplied part of the necessary cold to exchangers E2and E1. From the top of column C1 issues at G a gas mixture. Thismixture is reheated first in an exchanger H1 then in another exchanger11, before being compressed at 11. The compressed mixture is then cooledby exchanger 11, then enters, at K2, the high pressure column C2, afterhaving possibly undergone a partial expansion through a gate or valveB2. The column C2 is cooled at its upper portion by the above mentionedcondenser-vaporizer. A gaseous fraction consisting of the most volatileportions of the mixture fed to column C1 issues from the top of columnC2 through the tube L. The reflux in the column is obtained by thepartial condensation of the ascending gases in the condenser-vaporizer.

The liquid issuing from the bottom of column C2, through the tube M2, iscooled at H1 by a heat exchange with the gases issuing from the top ofthe column C1. It is expanded by the valve N and is used as a scrubbingliquid in the column C1.

The gases issuing from the head of the column C2 through the tube L areunited with the gases which escaped the liquefaction in the exchanger E2and arriving through the pipe P. The whole then undergoes a treatmentsimilar to the one just undergone by the initial mixture. It is cooledand partly liquefied in the exchanger E3. The liquid portion feeds,after being partially vaporized in a valve B3, a column C3, the bottomof which is heated by a condenser-vaporizer toppng a rectificationcolumn C4 operated at a higher pressure.

A liquid fraction is removed at the bottom of column Cs and issues fromthe apparatus at V, after being successively vaporized and warmed up inthe exchangers E3, E2, E1. From the top of column C2, issues at Q, a gasmixture which coolsfirstly an exchanger H2, then another exchanger I2before being compressed at J2. The compressed mixture is cooled inexchanger 12, then enters the column Ct at K4. Column C4 is cooled, atits upper portion, by the condenser-vaporizer already mentioned. Thegases issuing from the top of the column C4 are united with the gaseswhich escaped liquefaction in the exchanger E3. The whole enters at R anexchanger S, through which it passes three times, undergoing twointermediate expansions with external Work, in accordance with U. S.Patent No. 2,534,903 granted to the applicant. In exchanger S, due tothe cooling caused by the expansion, there is a partial liquefaction ofthe gas under pressure. The liquid so formed returns to the exchanger E3where it joins the liquid formed by the cooling, in said exchanger, ofthe gas from the first stage of separation, and feeding the column C3.The expanded gas cools in succession, firstly the exchanger S, then theexchangers E3, E2, E1, and finally issues at W.

Figure 2 shows a modified embodiment of the invention as applied to agas of mineral oil origin, with approximately the following composition:

Percent Propane 6.5 Propylene 8 Ethane 21 Ethylene 14.5 H2, CO, N2, CH4-50 The references of the parts common with the diagram of Figure l arethe same as in said figure, to which reference will be made for thedetails not mentioned hereinafter.

The gas mixture, compressed to 25 atmospheres is cooled to about C. inthe exchanger E1, then to about C. in the exchanger E2 wherein condensesa liquid rich in propane and propylene, which is treated in the columnC1 after expansion at B1, as indicated above in connection with Figurel. in contrast with the latter, the portion not condensed in theexchanger E2 is rectified in a column C5, the top of which is cooled bythe vaporization in a bundle of pipes T of the liquid fraction removedthrough line F and regulation valve B5, at the base of the column C1 andconsisting here in a propanepropylene mixture substantially free of themost volatile constituents. The liquid fraction resulting from therectification in the column C5 flows therefrom through the tube M5 backto the exchanger E2, While the gas fraction from this samerectification, is sent through the pipe X to the exchanger E3,undergoing therein a partial condensation as in the case of Figure l.

The portion liquefied in the exchanger E2 is expanded through the valveB1 to about 3.5 atmospheres, and rectified in the set of the two columnsC1 and C2, the latter being operated under a pressure of about 30atmospheres. Contrary to the diagram of Figure l, the gas portionresulting from the rectification in column C1 is sent through the tube Lin the column C4 which is operated under the same pressure. The rest ofthe apparatus is identical with that of Figure 1.

Upon the whole, the apparatus delivers:

At U a propane-propylene fraction.

At V an ethylene-ethane fraction.

At W the most volatile products, i. e. methane, carbon, oxide, nitrogen,and hydrogen.

Each one of these fractions may be treated by known means to beseparated into its constituents.

Instead of exchangers of the continuous operating type shown in thefigures, one can use, at least partly regenerators or exchangers of thecircuit inversion type.

Figure 3 shows diagrammatically a separation of air into oxygen, argonand nitrogen, utilizing the present invention. The air to be separated,previously cooled and under pressure, runs through a coil W arranged inthe bottom of high pressure column C'. Being so partially liquefied andproviding so the necessary heating of the column the air enters thecolumn C in A. In this column, the air is separated into a liquidfraction, holding about 45% of oxygen, which leaves column C at B andinto substantially pure nitrogen which is condensed in the upper part ofcolumn C in a condenser-reboiler B. Part of this nitrogen flows back incolumn C as scrubbing liquid, the remainder being withdrawn by pipe T3.

Column C is topped by a second column C" operated at atmosphericpressure, these columns being separated by the heat transfer unit E.Column C" is fed firstly at F with the 45% oxygen liquid removed at Bfrom the bottom of column C and expanded by valve V1, and then at itstop with liquid nitrogen withdrawn through pipe T3 from column C andexpanded by valve V3. Gaseous nitrogen issues by pipe 1 from the top ofcolumn C and gaseous oxygen by pipe S2 from the lower part of the samecolumn. In addition, a preferably liquid stream is extracted from columnC by a side pipe T at such a height that it is made with argon andoxygen. with substantially no nitrogen. This mixture enters at D in arectification column C1, operated at the same pressure as C. Column C isbound through a conventional condenser-vaporizer with a column C2operated at a higher pressure. The set of both columns C1 and C2operates acwrdin g to my invention.

In column C1, the argon-oxygen mixture separates into liquid oxygenwhich is withdrawn by a pipe T1 from the base F1 of column C1 and is ledback to column C, and into a gaseous argon-oxygen fraction issuing at Gfrom the top of column C1. This fraction is partly reheated in anexchanger H, then in another exchanger 1 before being compressed by acompressor I, then cooled in exchanger 1 and enters the bottom of columnC2 at K2. Column C2 is cooled at its top by the above mentionedcondenser-vaporizer ensuring the production of used as a scrubbingliquid in column C1.

In case the side stream extracted by pipe T from column C is in thegaseous state, the oxygen return from C1 to C" should be effected in thegaseous state too, through a pipe T2.

Instead of withdrawing from column C" through T a mixture of oxygen andargon, it is also possible though in that special case not soadvantageous, to withdraw from a higher level of C a mixture of argonand nitrogen and to submit it to treatment in columns C1 and C2. In sucha case, argon would be removed at P1 (being of course not returned tocolumn C), and nitrogen at L, being combined after a suitable expansionwith nitrogen issuing at S1 from column C.

What I claim is:

1. A method for low temperature separation of a gaseous mixture into atleast two fractions having different boiling points, by successivefractionation, in two rectification columns operating under differentpressures and provided with a heat transfer unit acting as a condenserfor the top of the higher pressure column and as a boiler for the bottomof the lower pressure column, which comprises, partly liquefying saidgaseous mixture and entering it in the lower pressure column, removingfrom the bottom of said column the higher boiling fraction and from itstop a gaseous efiluent, compressing the whole of said effluent andentering it in the higher pressure column, withdrawing from the top ofthe last said column the lower boiling fraction and withdrawing saidfraction in heat exchange with the incoming gaseous mixture to cool thesame and withdrawing from its bottom a liquid which is then expanded andused as a scrubbing liquid in the lower pressure column.

2. A method for low-temperature separation of a gaseous mixture into atleast three fractions having different boiling points, which comprisesseparating said mixture into two fractions according to claim 1 andrepeating the separation method according to claim 1 on either of thesetwo fractions.

3. A method of cold separating a gaseous mixture with three componentsof different volatilities into said components by rectification, whichmethod comprises, separating in a first stage said mixture into threefractions, one of which comprises substantially the whole of thecomponent with the middle volatility, and submitting the least namedfraction to the process according to claim 1.

4. A method of cold separating a gaseous mixture in three fractions ofdifferent volatilities, using two successive rectification columns atdifferent pressures separated from each other by a common heat transferunit functioning as a condenser for the vapors in the higher pressurecolumn and as a boiler for the liquid in the lower pressure column,which comprises, partly liquefying the said gaseous mixture by cooling,the portion remaining in the gaseous state after said liquefactionforming the first separated lowest boiling fraction, entering theresulting liquid after a partial expansion in a first rectificationcolumn operating under a relatively low pressure, removing from thebottom of said column the second separated highest boiling fraction andfrom the top a gaseous efliuent, compressing this gaseous efliuent andentering it in a second rectification column operating under arelatively high pressure and in heat exchange with the firstrectification column, removing from the top of this column a gaseousefliuent making up the third separated middle boiling fraction, and fromthe bottom of this same column a liquid which is then expanded and usedas a scrubbing liquid in the lower pressure rectification column.

5. A method of cold separating a gaseous mixture in several fractions ofdiiferent volatilities, which comprises separating it in three fractionsaccording to claim 4, joining the first and third fractions andrepeating on said joined fractions the method according to claim 4.

6. A method of cold separating a gaseous mixture according to claim 4,which comprises, expanding the gaseous lowest boiling fraction and usingthis cold expanded fraction for the cooling of the mixture to beseparated.

7. A method of cold separating a gaseous mixture in several fractions ofdifferent volatilities which comprises separating it in three fractionsaccording to claim 4, submitting the gaseous portion remaining after thepartial liquefaction of the initial mixture to a rectification usingindirect contact with the expanded liquid second separated fraction, theliquid resulting from said rectification being used for cooling theinitial mixture and thereafter collected for treatment according toclaim 4, the gaseous effluent from said rectification being alsosubmitted to a process according to claim 4.

8. A method of cold separating a gaseous mixture in more than threefractions of different volatilities, which comprises separating it inthree fractions according to claim 4 and repeating the method accordingto claim 4 on the separated lowest boiling fraction.

9. A method of cold separating a gaseous mixture in more than threefractions of different volatilities, which comprises separating it inthree fractions according to claim 4 and repeating the method accordingto claim 4 on the separated middle boiling fraction.

10. A method of cold separating a gaseous mixture which comprises,cooling and partly liquefying this mix ture, expanding the liquidportion, separating in two fractions the resulting mixture of gas andliquid by successive fractionation in two rectification columnsoperating under different pressures and provided with a heat transferunit acting as a condenser for the top of the higher pressure column andas a boiler for the bottom of the lower pressure column, saidfractionation comprising the steps, entering said liquefied portion ofthe gaseous mixture into the lower-pressure column, removing from thebottom of said column the higher boiling fraction and from its top agaseous effluent, compressing the whole of said effluent and entering itin the higher pressure column, withdrawing from the top of the last saidcolumn the gaseous lower boiling fraction and from its bottom a liquidwhich is then expanded and used as a scrubbing liquid in the lowerpressure column, and which comprises further, expanding said gaseouslower boiling fraction resulting from this last separation and usingthis cold expanded fraction for the cooling of the initial mixture.

References Cited in the file of this patent UNITED STATES PATENTS

1. A METHOD FOR LOW TEMPERATURE SEPARATION OF A GASEOUS MIXTURE INTO ATLEAST TWO FRACTIONS HAVING DIFFERENT BOILING POINTS, BY SUCCESSIVEFRACTIONATION, IN TWO RECTIFRICATION COLUMNS OPERATING UNDER DIFFERENTPRESSURES AND PROVIDED WITH A HEAT TRANSFER UNIT ACTING AS A CONDENSERFOR THE TOP OF THE HIGHER PRESSURE COLUMN AND AS A BOILER FOR THE BOTTOMOF THE LOW PRESSURE COLUMN, WHICH COMPRISES, PARTLY LIQUEFYING SAIDGASEOUS MIXTURE AND ENTERING IT IN THE LOWER PRESSURE COLUMN, REMOVINGFROM THE BOTTOM OF SAID COLUMN THE HIGHER BOILING FRACTION AND FROM ITSTOP A GASEOUS EFFLUENT, COMPRESSING THE WHOLE OF SAID EFFLUENT ANDENTERING IT IN THE HIGHER PRESSURE COLUMN, WITHDRAWING FORM THE TOP OFTHE LAST SAID COLUM THE LOWER BOILING FRACTION AND WITHDRAWING SAIDFRACTION IN HEAT EXCHANGE WITH THE INCOMING GASEOUS MIXTURE TO COOL THESAME AND WITHDRAWING FORM ITS BOTTOM A LIQUID WHICH IS THEN EXPANDED ANDUSED AS A SCRUBBING LIQUID IN THE LOWER PRESSURE COLUMN.